1. Field of Invention
The present invention relates to high temperature heat recovery systems. More particularly, the invention is a high temperature heat exchanger made of refractory and ceramic materials which can operate in gas streams of up to about 1,500.degree. C.
2. Description of the Related Art
Furnaces for transferring heat to processed products at very high temperatures (such as for glass or metal smelting) result in stack gas temperatures at approximately the temperature of the furnace bed. In addition to fuel which must be added to the furnace to melt the product, fuel must also be added to heat the combustion air up to the average temperature in the furnace. Without some type of heat recovery system in the stacks, the energy associated with stack gases is lost when the gases are vented out of the system at furnace temperature. Heat exchangers are therefore routinely used to recover stack gas enthalpy by transferring this heat to the incoming combustion air.
However, the ultra high temperatures of operation create a highly erosive and destructive environment for most engineering materials including steel. There remains a need in the art for a heat exchanger capable of efficient operation at such elevated temperatures.
All metal heat exchangers are known such as disclosed in British Patent No. 191,175 issued to Walker, which describes a metal gas cooler having a double tube sheet design in which gas tubes are disposed between two walls of each tube sheet. Process gas is designed to pass through the tubes while cooling liquid passes over the outside of the tubes. The stated purpose of the two walls is to ensure that none of the cooling liquid comes in contact with the process gas. Such heat exchangers are temperature limited due to the all metal construction and are incapable of operating in hot gas streams above 850.degree. C. for extended periods of time without inevitable tube failures.
Attempts have been made to develop designs for high temperature heat exchangers using tubes and tube sheets made of ceramics or refractories or a combination of like materials. Known ceramic heat exchangers have had limited success due to failures in three areas.
The first is in the tubes where complications occur due to oxidation or chemical deterioration of the ceramic materials which cause the tubes to break. Another mode of tube failure is a phenomenon known as thermal shock which happens if the ceramic tube is heated or cooled too quickly, above an acceptable rate.
The second area of failure is in the seal between the tube and the tube sheet. Currently available ceramic heat exchangers have ceramic fiber materials which form a seal between the outside surface of the tube and the inside surface of the tube sheet. In some cases the tube end is also designed to press against a ceramic fiber ring which in turn is held in a relief cut into the tube opening in the tube sheet. When such ceramic heat exchangers are cycled between hot and cold conditions, the tubes and tube sheets expand and contract. Expansion of the tube compresses or deforms the ceramic fiber seal which has no memory of its original shape. After a few cycles, the seal material rapidly disintegrates into powder which blows out of the seal area.
Some ceramic fiber materials can also melt in the seal at extreme temperatures or after chemical exposure from the process. During the next cold cycle, as the tube contracts and pulls back from the seal, the molten seal material fills the resulting void and hardens. When these types of ceramic heat exchangers are brought back up to operating temperature, the tube expands before the deformed seal material can melt out of the way. This causes the tube to fail or makes the tube push the tube sheet out of position resulting in tube sheet leaks.
The third failure mode comes from conventional tube sheet designs, the most common of which require refractory blocks or ceramic tiles stacked one above the other to form the tube sheet. In some designs there are interlocking notches or grooves which have alignment keys or ceramic fiber material to seal the joints between adjacent blocks or tiles. Such tube sheets expand and contract with the hot and cold temperature cycles of the process with differential expansion from the hot side to the cold side as the tubes absorb the heat from the process stream.
For example, if the process flow enters such heat exchangers at the bottom of the tube sheet at 1,400.degree. C., it can exit the heat exchanger from the top of the tube sheet at 1,200.degree. C., causing a reduced rate of thermal expansion at the top of the tube sheet relative to the bottom. This differential expansion can break the bonds between the tiles making up the tube sheet and cause misalignment between the tube sheets of the ceramic heat exchanger which in turn can bind or break the tubes.
Problems also exist in the Joints and seams which hold together the refractory blocks or ceramic tiles. The joints and seams usually have a binder to hold the tiles in place. The problem is that tubes are much stronger than currently known binder material design. When tube seals melt or degrade, the expanding tubes can push against the tube sheet and break the tile binder which will destroy the tube sheet integrity and cause it to leak.
Leaks can also be caused by eutectic formations on the hot gas side of tube sheets made of refractory blocks or ceramic tiles. Vaporized chemicals in the hot gas can attack the tube side surface of the tube sheet and cause a reduction in the melting temperature of the refractory block or ceramic tile surfaces. At hot operating temperatures, the surface of such tube sheets can melt, and this molten material from the surface can enter cracks which develop in the binder between the tiles. At the next cool down cycle, the molten material within the cracks solidifies and the tube sheet is unable to return to its original shape. Over time this will distort the tube sheet and cause tube misalignment resulting in tube bindings and breaks.
Attempts at developing a high temperature ceramic heat exchanger have been published. For instance, U.S. Pat. No. 4,632,181 issued to Graham describes a ceramic heat exchanger of a tube and shell design with a tube sheet made from a series of stacked tiles. The Graham patent is typical of the state of the art, it describes tubes, tube seals and tube sheet walls which suffer weaknesses as discussed above.
U.S. Pat. No. 4,449,575 issued to Laws describes a heat transfer system in which ceramic tubes are mounted in a fluid bed reactor furnace. However, the Laws design requires a metal locking means for compressing a fibrous ceramic seal against the ceramic tube which limits it to operating temperatures below 900.degree. C. The ceramic fiber seals also degrade over time and expanding tubes cause a permanent deflection of the packing as described above.
British Patent Application No. GB2015146A by Laws describes a tube and shell heat exchanger having ceramic tubes mounted into a wall made from parallel elongated ceramic tube blocks held in place by metal bolts. The metal bolts limit this system to operating temperatures below 900.degree. C., and the walls held by the bolts develop leaks through the seams which expand with ash buildup over time.
U.S. Pat. No. 4,122,894 issued to Laws et al. describes another conventional ceramic tube and shell heat exchanger having a tapered tube opening in the tube sheet into which different diameter packing rings concentric with the tube can be inserted. This design also requires a ceramic fiber seal which tightly restrains the tube and which are prone to leakage as the tube expands into the tube sheet, as noted above.
U.S. Pat. No. 4,106,556 issued to Heym et al. describes a ceramic heat exchanger system where two tube sheets are mounted one above the other on the same (top) side of a hot gas duct. The Heym tube sheets must have metal plates on surfaces exposed to the process gas and all tubes are held in their respective tube sheets with metal lock rings which, again, limits the design to operating temperatures below 900.degree. C.
U.S. Pat. No. 3,923,314 issued to Lawlet et al. describes a heat exchanger made of silicon carbide tubes mounted in a silicon carbide tube sheet. The silicon carbide exchanger is specifically designed for heating aqueous, highly corrosive, acid streams at about 205.degree. C. and is not suitable for high temperature heat exchange.
There is a need in the art for a practical design which corrects the deficiencies hereinabove noted and allows a heat exchanger to operate at very high temperatures without tube, tube seal or tube sheet failures. Currently available ceramic heat exchangers have tube and tube sheet seal problems due to a lack of freedom in physical mobility which this disclosure identifies as a requirement to protect the system against large thermal expansions and contractions experienced during operation cycles between low and high temperature operation.